1. Field of the Invention
This invention relates to ionic hydrocarbon copolymers and more specifically it relates to zinc neutralized ionic copolymers having improved adhesion to nylon.
2. Description of the Prior Art
Multi-layer structures consisting of two or more plastics provide unusual combination of barrier and mechanical properties necessary for todays packaging applications. There are many methods of producing multi-layered film structures. The two basic techniques are adhesive laminating and coextrusion. The adhesive lamination technique requires that part or all of the various component film layers be produced separately, and subsequently joined by heat and pressure, or by the addition of an adhesive layer between films or both. A portion of the total composite can be produced by coextrusion and subsequently joined to the other films as described above.
Coextrusion is the process of simultaneously extruding two or more materials from one die. The majority of coextrusion mechanisms combine the two or more polymer flows in the die; there are methods of combining these layers immediately upon exiting the die. A successful coextrusion operation must consider the viscosities of the various components. They must be reasonably matched to avoid unacceptable gauge variation. Viscosity matching in coextrusion is well-known in the art and is practiced commercially.
An equally important consideration is the mutual adhesion of the layers that occurs under temperature and pressure conditions prevailing in the die. The required level of adhesion is dependent on the end-use application; most require high levels of layer to layer adhesion. Generally speaking, polymers with similar chemical structures readily adhere, for example, low density polyethylene and high density polyethylene. Dissimilar materials, however, show poor adhesion. Examples are high density polyethylene and polystyrene and low density polyethylene with nylon.
Dissimilar materials can be bonded together with the use of an extrudable adhesive layer between the two dissimilar materials. A specific material must be chosen which has adhesion to both substrates. A broad selection of these materials exist; the more widely used adhesive materials are ethylene/vinyl acetate, ethylene/ethyl acrylate, and ionomers. The ability to coextrude ionomers and nylon to produce packaging films is a well-known art and is practiced commercially. Commercially available ionomers coextruded with nylon generally produce composites with poor to marginal adhesion. The level of adhesion is dependent upon the ionomer of choice, the nylon of choice, the temperature at which the two polymers are married, the hold-up time in the die, and the pressure in the die. Under optimum conditions of time, temperature and pressure ionomer/nylon composites with good bonding can be produced but these conditions are not economically attractive for commercial coextrusion.
The mechanism of adhesion between similar materials such as high density polyethylene and low density polyethylene is thought to be cocrystallization at the interface of the two polymers. The mechanism of adhesion between ionomer resins and nylon is considerably more complex and is not well understood. One theory of adhesion is the formation of an amide chelate.
Rees U.S. Pat. No. 3,264,272 broadly discloses and claims ionic copolymers of at least 50 mol percent .alpha.-olefins and 0.2 to 25 mol percent .alpha.,.beta.-ethylenically unsaturated carboxylic acids (and optionally third ethylenically unsaturated comonomers) wherein 10 to 90 percent of the carboxylic acid groups are neutralized with metal ions such as sodium, potassium, silver, mercury, magnesium, calcium, barium, iron, zinc, aluminum, etc. There is no teaching in Rees regarding adhesion to and coextrusion with nylon or the special characteristics of the ionomer within a specific narrow range of salt content.
Goehring U.S. Pat. No. 3,791,915 discloses a laminate film consisting of a polyamide polymer adhered to a blend of polyethylene and zinc neutralized ionomer by a zinc neutralized ionomer. Goehring points out that contrary to what might be expected, ionomers cannot be broadly used as adhesives; but are selective as to the substrates to which they will adhere. He reports the the fact that sodium ionomers do not adhere to nylon, while zinc ionomers do adhere.
Using Surylyn.RTM. ionomer resins 1650 and 1800, he observed good Surlyn.RTM./nylon adhesion in a 3 layer coextruded tube (5 mils nylon, 8 mils Surlyn.RTM., 8 mils polyethylene). The good adhesion observed is a function of the thickness of the tube and the conditions of temperature and pressure at which it was produced. In commercial blown film and cast film lines composite barrier films, because of their cost, are made as thin as possible, especially with regard to the higher priced materials present. Composites of 4 mils or less made at maximum line speed are the rule.
Our Surlyn.RTM./nylon coextrusion experiments included Surlyn.RTM. 1650 and 1800, the two ionomers mentioned by Goehring as giving good adhesion to nylon. To a standard type 6 nylon polymer ("Allied" 8207) these ionomers gave 250 grams per inch and 110 grams per inch adhesion, respectively. Under the conditions of coextrusion we observed enhanced bonding to nylon over the narrow salt range of the present invention (cf. Examples) which range does not include either of these two resins employed by Goehring.